This invention relates to the coating of optical waveguide fibers.
In the manufacture of optical waveguide fibers, a preferred method for manufacturing the fibers is i) produce a preform using deposition techniques such as outside vapor deposition (OVD), vapor axial deposition (VAD) or modified chemical vapor deposition (MCVD); ii) dehydrate and consolidate the OVD or VAD soot preform (or, collapse the MCVD preform) to produce a solid glass preform; and iii) draw the glass preform into fiber. Because of the requirements of high strength and low loss, a protective coating is applied to the drawn fiber before the pristine surface of the fiber is damaged by subsequent handling, either during manufacture or subsequent use. This coating step is typically performed as an integral part of the drawing process to ensure that the coating material is applied before the surface of the fiber is damaged. Optical fibers are also combined in arrays, known as ribbon fibers, the manufacture of which requires that an overcoating be applied to an array of coated or uncoated optical fibers.
A coating material commonly used in the manufacture of optical waveguide fibers is an acrylate-based composition which is curable by exposure to ultraviolet (UV) light. This material is applied to the surface of the fiber in a liquid state and is subsequently exposed to UV light for curing. The coating material may be applied in one or more layers, with a two-layer coating system being a preferred embodiment. The first, or primary, coating is applied directly to the surface of the fiber, and the second, or secondary, coating is applied over the primary coating.
When coating an optical waveguide fiber, it is important to produce, at high drawing rates, coated fibers with consistent coated diameter and coatings which are applied concentrically to the fiber. Both of these attributes contribute to ease in splicing and connectorization of the fiber, thereby providing for lower losses in an installed fiber application. The higher draw rates reduce the cost of manufacturing the fiber by increasing output and equipment utilization. Market demands continue to place more stringent tolerances on the coated diameter of an optical waveguide fiber. Present manufacturing processes provide coatings with tolerances of about .+-.15 .mu.m, while tolerances in the range of about .+-.3-5 .mu.m are needed.
Much work has been directed toward cooling the drawn fiber prior to receiving the first layer of coating material. This is required because the high drawing temperatures (in the range of 1,800.degree. to 2,000.degree. C.) and increasing drawing rates would result in the temperature of the fiber at the first coating application being too high for adequate coating to occur. If the fiber temperature is too high when the coating is applied, the quality, dimensions, and consistency of the coating thus applied would be adversely affected. It is generally known that the fiber temperature should be less than about 300.degree. C. for proper application of the coating material. Pack et al., "Forced Convective Cooling of Optical Fibers in High Speed Coating", J. of Applied Physics, vol. 50, no. 10, pp. 6144-48, Oct. 1979. Examples of systems designed to reduce the temperature of the fiber prior to the first application of coating include:
Paek et al. U.S. Pat. No. 4,594,088 which discloses a fiber drawing system which includes a cooling chamber containing a liquid heat transfer medium through which the fiber is drawn prior to application of the coating material;
Darcangelo et al. U.S. Pat. No. 4,514,205 which discloses an elongated coolant tube into which both a hot drawn fiber and cooled helium are introduced, and the cooled helium reduces the temperature of the fiber prior to application of the coating material; and
Cain et al. U.S. Pat. No. 5,043,001 which discloses a cooling chamber which contains a liquid through which a hot fiber passes and is cooled thereby, the cooled fiber not being wetted by the liquid because a vapor barrier is formed at the surface of the fiber due to boiling of the liquid at the fiber surface.
A fiber drawing and coating system, as currently used in the production of optical waveguide fibers, is shown in FIG. 1. Fiber 10 is drawn from preform 11 which is heated in furnace 1. Fiber 10 then passes through fiber cooling device 2 and is cooled to a temperature below about 90.degree. C. Fiber 10 then passes through primary coater assembly 3 and is coated with a primary layer of coating. The primary coating layer is cured in primary coating curing device 4, and the diameter of the fiber including the cured primary coating is measured by device 5. Curing device 4 typically comprises an irradiator array. Fiber 10 then passes through secondary coater assembly 6 and is coated with a secondary layer of coating. The secondary coating layer is cured in secondary coating curing device 7, which is similar to primary coating curing device 4, and the diameter of the fiber including the cured secondary coating is measured by device 8. Tractor means 9 is used to pull the fiber from furnace 1 through the intermediate devices. The fiber is then typically taken up onto spools by a winder (not shown) for further processing.
FIG. 2 is a more detailed view of a coating die assembly. Fiber 21 enters coating die assembly 20 through guide die 22. Coating material is delivered to coating die assembly 20 through holes 24 in insert 23. The coating material is delivered under pressure at a constant temperature. Fiber 21 exits coating die assembly 20 through sizing die 25. As fiber 21 passes through coating die assembly 20, the coating material is accelerated. As the coating material and fiber 21 enter sizing die 25, a portion of the coating material is pulled out with the fiber. The coating material that is accelerated by the fiber, but not pulled out with the fiber, recirculates within coating die assembly 20. Coating die assembly 20, as shown in FIG. 2, is similar to that disclosed in Kar U.S. Pat. No. 4,531,959, the relevant portions of which are incorporated herein by reference. The coating is applied using a method which reduces the formation of bubbles in the coating as disclosed in Deneka et at. U.S. Pat. No. 4,792,347, the relevant portions of which are incorporated herein by reference.
The amount of coating material which is drawn out with fiber 21 is dependent on the velocity profile of the coating material within sizing die 25. This velocity profile is most affected by the speed at which fiber 21 is drawn through coating die assembly 20, the geometry of sizing die 25 and the viscosity profile of the coating material in sizing die 25. The viscosity profile of the coating material is a function of its temperature, which is influenced by: i) the temperature of fiber 21; ii) the temperature of the walls of sizing die 25; iii) internal heat generation known as "viscous heating" which is the result of the conversion of mechanical energy to thermal energy via fluid friction; iv) the temperature of the incoming coating; and v) the temperature of any surface with which the coating thermally communicates. The viscosity profile can also be a function of shear rate or, equivalently, an applied stress. Fluids such as these are described as non-Newtonian.
For a given coating die assembly geometry, the coated fiber diameter is determined by the velocity profile within the coating material at the exit of sizing die 25. The velocity profile at the exit of sizing die 25 can be affected by the velocity profile at other parts of coating die assembly 20. Therefore, the region at or near which the coated fiber diameter is determined can include any portion of coating die assembly 20 in which the velocity profile can be affected such that the velocity profile at the exit of sizing die 25 is also affected. This region can include the entire sizing die, or even portions of coating die assembly 20 near insert 23, if the control of the coating material temperature, and, therefore, the viscosity profile, can be sufficient to provide adequate control of the coated fiber diameter. However, as noted below, we have found that when the region in which the viscosity profile is controlled, by controlling the temperature of the coating material, is localized to land region 26 of sizing die 25, one can achieve very responsive control of coated fiber diameter.
Referring to reference numbers in FIG. 1, while it is possible to control the temperature of fiber 10 after primary coating device 4 and prior to secondary coater assembly 6, it is generally neither convenient nor practical to do so. For example, it takes either a long residence time in a low gas flow temperature adjusting apparatus or a high gas flow in a short residence time temperature adjusting apparatus to change the temperature of the fiber. This is due to the relatively high thermal mass of the fiber in combination with the primary coating layer (approximately three times the thermal mass as compared to an uncoated fiber). There are space limitations on the fiber drawing apparatus which prevent the use of a long residence time temperature adjusting apparatus. Also, changing the temperature of the fiber may have adverse affects on subsequent steps in the fiber drawing process.
Albarino et al. U.S. Pat. No. 4,073,974 discloses a coating system which includes a coil for heating the coating material prior to delivery to a sizing die. The heating coil is used to adjust the temperature of the coating material, thereby adjusting the viscosity of the coating material. The heating coil, however, is located a significant distance from the sizing die of the coating system which is located near the point at which the fiber exits the coating system. Therefore, the effect of the heating coil on the viscosity of the coating material at the sizing die is, at best, unpredictable. We believe this is particularly true because of the effects of viscous heating described previously. There is no disclosure in Albarino et al. of any measurement of coated fiber diameter or use of any such measurement to control the heating coil.
Mackay U.S. Pat. No. 4,622,242 discloses a pressurized coating material applicator die in which a single heat exchange "circuit" is used to control the temperature of a coating material in a coating supply tube and, optionally, a coating die chamber. The system disclosed in Mackay requires that, if the coating die chamber is included in the "circuit", the temperature of the coating material in the coating supply tube and in the coating die chamber are the same. The temperature of the coating material is predetermined based on the particular coating material used. There is no disclosure in Mackay of any feedback control of the die temperature based on coated fiber diameter. Also, the system disclosed in Mackay affects only the macroscopic temperature of the entire applicator die. These limitations will adversely impact the coated fiber diameter.
Hosotani et al. Japanese Patent Publication No. 63-74938 discloses a coating system which includes a variable-temperature gas temperature adjusting apparatus used to modify the temperature of a fiber after the primary coating layer has been applied to the fiber and cured. Hosotani et al. discloses that by controlling the fiber temperature prior to application of the secondary coating, the viscosity distribution of the secondary coating is affected which will in turn affect the amount of secondary coating applied to the fiber.
The system disclosed in Hosotani et al. is shown in FIG. 3. Fiber 33 is drawn from preform 31, which is heated in furnace 32. The fiber is coated with a primary coating layer as it passes through primary coater assembly 34. The primary coating layer is cured in curing device 35. The temperature of the fiber with the cured primary coating is adjusted by temperature adjusting apparatus 36 prior to entering secondary coater assembly 37. The secondary coating is cured in curing device 38, and the diameter of the fiber with both the secondary and primary coatings is measured by diameter measuring device 39.
In Hosotani et al., the temperature of the fiber is adjusted prior to the application of the secondary coating based on the diameter of the coated fiber (both the primary and secondary coatings). Temperature adjusting apparatus 36 blows a gas, such as N.sub.2, on the fiber to change the temperature of the fiber. The gas temperature is changed using a heater or cooling device as required to obtain the desired fiber temperature. The system disclosed in Hosotani et al. has several problems. First, the examples disclosed in Hosotani et at. are for fiber drawing rates in a range of about 0.8 to 1.7 m/sec. We believe that the system would require large quantities of contaminant and particulate free gas to affect the temperature of the primary coated fiber at the higher drawing speeds, for example, faster than about 5 m/sec, which are typical in current production of optical waveguide fibers. Second, even if the temperature of the primary coated fiber can be affected using the system disclosed in Hosotani et al., the speed at which such a system reacts to short-term variations in coated fiber diameter will be substantially slower than required for tight tolerances on coated fiber diameter.